Lead calcium tin alloys have been used as electrowinning anodes for copper electrowinning for many years. Prengaman et al. in U.S. Pat. No. 4,373,654 developed the first rolled lead calcium tin anode. These anodes have been used in copper electrowinning service since the early 1980's. The anodes utilizing rolled lead calcium tin alloys have a long life. The combination of calcium and tin content along with mechanical working produced a material with high mechanical strength to prevent distortion, warping and short circuits while in service. The combination of tin and calcium reduces the rate of corrosion, promotes the formation of a conductive corrosion layer on the anode surface and improves the stability of the anode leading to improved anode life. Improvements have been made by Prengaman in the attachment of rolled alloy sheets to the copper bus bar in U.S. Pat. No. 6,131,798. In Prengaman et al. U.S. Pat. No. 5,172,850, the copper bus bar is protected from attack by coating it with a layer of electro-deposited lead onto the copper bus bar, thus improving the resistance to acid.
Despite the improvements in life in the copper electrowinning anodes, the anodes are corroded by the oxygen generated in the electrowinning process. Prengaman in “Improved Copper Electrowinning Operations Using Wrought Pb, Ca, Sn Anodes,” Cu 99 International Symposium, October 1999, describes the anode corrosion. Oxygen is either evolved as oxygen gas or diffuses through the corrosion product on the surface of the anode to the lead surface where it reacts with the lead alloy to corrode the anode. It is important to produce a complete uniform, compact, thin, adherent and conductive PbO2 corrosion layer on the surface of the anode so that the oxygen can be evolved efficiently.
As the corrosion product becomes thicker, it begins to develop small cracks parallel to the anode surface. These cracks eventually result in the production of non-adherent flakes on the surface of the anode. The corrosion product can then be dislodged from the surface by the bubbles of oxygen generated at the anode surface. If the flakes contact the cathode, they can be reduced to metallic lead and become entrained in the cathode.
The rate of corrosion is related to the electrolyte temperature and current density of the electrowinning cell. The higher the current density and the higher the temperature, the more rapid is the rate of corrosion. In addition to the electrowinning cell conditions, the electrolyte often contains manganese. Manganese can react with the PbO2 corrosion product on the surface of the oxide, making it less stable and adherent and thus more susceptible to shedding. This was described by Prengaman in Cu 87 volume 3 and Electrometallurgy of copper Ed by W. Cooper, G. Loyas, G. Vearte, p. 387.
To reduce the rate of corrosion of the anode, increase the oxygen evolution and reduce the deleterious effects of the manganese, cobalt has been added to copper electrowinning electrolytes. Cobalt addition to electrowinning solutions was first described by O. Hyvarinen, P&D thesis 1971 and more recently by Yu and O'Keefe in J. Electrochem Society 146 (4) 1999, p. 1361, “Evolution of Lead Anode Reactions in Acid Sulfate Electrolytes I. Lead Anodes with Cobalt Additives.”
The cobalt depolarizes the oxygen evolution reaction leading to easier oxygen evolution. This results in reduced anode corrosion, improved copper cathode quality and longer anode life. Cobalt ions are absorbed onto the lead corrosion product. Analysis of the corrosion product shows the presence of cobalt.
Cobalt is added to the electrolyte in an amount of generally 50-300 ppm. Jenkins et al., in copper 99 Vol. IV Hydrometallurgy of Copper Electrolyte Copper-Leach, Solvent Extraction and Electrowinning World Operation Data, surveys the operating conditions from 34 copper electrowinning tankhouses. To maintain the cobalt content of the electrolyte, cobalt must be continuously added to make up for the bleed of electrolyte from this system to control the impurities in the electrolyte. The cobalt addition varies from 100-800 g per ton of copper cathode. Loss of cobalt in the bleed is a major cost in operating copper tankhouse.